DESCRIPTION

Mbuf pools are intended to help drivers for interface cards that need
huge amounts of receive buffers, and additionally provides a mapping
between these buffers and 32-bit handles.
An example of these cards are the Fore/Marconi ForeRunnerHE cards. These
employ up to 8 receive groups, each with two buffer pools, each of which
can contain up to 8192. This gives a total maximum number of more than
100000 buffers. Even with a more moderate configuration the card eats
several thousand buffers. Each of these buffers must be mapped for DMA.
While for machines without an IOMMU and with lesser than 4GByte memory
this is not a problem, for other machines this may quickly eat up all
available IOMMU address space and/or bounce buffers. On sparc64, the
default I/O page size is 16k, so mapping a simple mbuf wastes 31/32 of
the address space.
Another problem with most of these cards is that they support putting a
32-bit handle into the buffer descriptor together with the physical
address. This handle is reflected back to the driver when the buffer is
filled, and assists the driver in finding the buffer in host memory. For
32-bit machines, the virtual address of the buffer is usually used as the
handle. This does not work for 64-bit machines for obvious reasons, so a
mapping is needed between these handles and the buffers. This mapping
should be possible without searching lists and the like.
An mbuf pool overcomes both problems by allocating DMA-able memory page
wise with a per-pool configurable page size. Each page is divided into a
number of equally-sized chunks, the last MBPOOL_TRAILER_SIZE of which are
used by the pool code (4 bytes). The rest of each chunk is usable as a
buffer. There is a per-pool limit on pages that will be allocated.
Additionally, the code manages two flags for each buffer: ``on-card'' and
``used''. A buffer may be in one of three states:
free None of the flags is set.
on-card Both flags are set. The buffer is assumed to be handed over to
the card and waiting to be filled.
used The buffer was returned by the card and is now travelling
through the system.
A pool is created with mbp_create(). This call specifies a DMA tag dmat
to be used to create and map the memory pages via bus_dmamem_alloc(9).
The chunk_size includes the pool overhead. It means that to get buffers
for 5 ATM cells (240 bytes), a chunk size of 256 should be specified.
This results in 12 unused bytes between the buffer, and the pool overhead
of four byte. The total maximum number of buffers in a pool is max_pages
* (page_size / chunk_size). The maximum value for max_pages is 2^14-1
(16383) and the maximum of page_size / chunk_size is 2^9 (512). If the
call is successful, a pointer to a newly allocated structmbpool is set
into the variable pointed to by mpb.
A pool is destroyed with mbp_destroy(). This frees all pages and the
pool structure itself. If compiled with DIAGNOSTICS, the code checks
that all buffers are free. If not, a warning message is issued to the
console.
A buffer is allocated with mbp_alloc(). This returns the virtual address
of the buffer and stores the physical address into the variable pointed
to by pa. The handle is stored into the variable pointed to by hp. The
two most significant bits and the 7 least significant bits of the handle
are unused by the pool code and may be used by the caller. These are
automatically stripped when passing a handle to one of the other
functions. If a buffer cannot be allocated (either because the maximum
number of pages is reached, no memory is available or the memory cannot
be mapped), NULL is returned. If a buffer could be allocated, it is in
the ``on-card'' state.
When the buffer is returned by the card, the driver calls mbp_get() with
the handle. This function returns the virtual address of the buffer and
clears the ``on-card'' bit. The buffer is now in the ``used'' state.
The function mbp_get_keep() differs from mbp_get() in that it does not
clear the ``on-card'' bit. This can be used for buffers that are
returned ``partially'' by the card.
A buffer is freed by calling mbp_free() with the virtual address of the
buffer. This clears the ``used'' bit, and puts the buffer on the free
list of the pool. Note that free buffers are NOT returned to the system.
The function mbp_ext_free() can be given to m_extadd() as the free
function. The user argument must be the pointer to the pool.
Before using the contents of a buffer returned by the card, the driver
must call mbp_sync() with the appropriate parameters. This results in a
call to bus_dmamap_sync(9) for the buffer.
All buffers in the pool that are currently in the ``on-card'' state can
be freed with a call to mbp_card_free(). This may be called by the
driver when it stops the interface. Buffers in the ``used'' state are
not freed by this call.
For debugging it is possible to call mbp_count(). This returns the
number of buffers in the ``used'' and ``on-card'' states and the number
of buffers on the free list.